The invention relates to a master disk for magnetic printing and a method of manufacturing the same. More particular, the invention is directed to a master disk and its method of manufacturing for magnetic printing, provided so as to write servo signals for positioning a head or specified data onto a surface of a magnetic disk using magnetic printing technology. The head carries out writing of data to/reading of written data from the surface of the magnetic recording disk in a hard disk drive (hereinafter referred to as an “HDD”), which drive is currently mainstream as external computer storage. The magnetic recording disk has as a recording material at its surface a magnetic film.
In the above-described HDD, recording and reproducing of data are carried out while a magnetic head floats on a surface of a rotating magnetic disk as a magnetic recording medium, kept there several tens nanometers from the surface of the disk by a floating mechanism known as a slider. On the magnetic recording medium, bit information is stored in data tracks arranged in concentric circles on the magnetic recording medium. The data recording/reproducing head is moved and positioned at a high speed onto a target data track on the magnetic recording medium to record and reproduce the data.
On the surface of the magnetic recording medium, positioning signals (servo signals) for detecting a position of the head relative to the data track, are written in concentric circles. This allows the head carrying out the recordation and reproduction of data to detect its own position at fixed time intervals. The servo signal is written by using a specialized device known as a servo writer after the magnetic recording medium is mounted in an HDD device, so that the center of the written servo signal causes no eccentricity to the center of the magnetic recording medium (or the center of the path (orbit) of the head).
A recording density in a present stage of development has reached up to 100 Gbits/in2 and, along with this, the recording density is increasing at an annual rate of 60%. Accompanying this, the density of the servo signal used by the head for detecting its own position is increasing, and the time for writing the servo signal has also tended to increase year by year. An accompanying increase in the writing time of the servo signal has become one of the major causes of reduction in manufacturing productivity and an increase in the cost of the HDDs.
Recently, in contrast with the above-described method of writing the servo signal using a writing head of the servo writer, a technological development has occurred concerning a method for dramatically reducing the writing time of servo information. This involves writing the servo signal in a lump by magnetic printing.
FIGS. 3(a) to 3(c) and FIGS. 4(a) and 4(b) are views for explaining a magnetic printing technology. FIGS. 3(a) to 3(c) are views for explaining process steps of magnetic printing in a magnetic recording medium. FIGS. 4(a) and 4(b) are views for explaining the principle of the magnetic printing in the magnetic recording medium.
FIG. 4(a) shows from the direction of a cross section of a substrate, a state in which a permanent magnet with magnetic field flux lines in a direction MFD moves in a direction TD on the surface of a magnetic recording medium while keeping a fixed distance (one mm or less) therefrom. A magnetic film 42 deposited on a substrate 41 forming a magnetic recording medium 40, initially is not magnetized in a definite direction. However, the film 42 becomes magnetized in a definite direction by a leakage magnetic flux emanating from a gap of a permanent magnet 43 (arrows drawn in the magnetic film in the drawing indicate the direction of magnetization). The step is here referred to as an initial demagnetization step.
The initial demagnetization step shown in FIG. 4(a) corresponds to an initial demagnetization step shown in FIG. 3(a). An arrow in FIG. 3(a) represents the moving path of the permanent magnet 43 in FIG. 4(a), by which the magnetic film 42 is uniformly magnetized. FIG. 3(b) shows a master disk for magnetic printing (hereinafter referred to as a “master disk”) laid on the magnetic recording medium to carry out positioning. FIG. 3(c) shows a state of carrying out magnetic printing by bringing the master disk into intimate contact with the surface of the magnetic recording medium and by moving the permanent magnet for magnetic printing along the moving path illustrated by an arrow in the figure.
Moreover, FIG. 4(b) shows the step of writing a printing pattern, and corresponds to FIG. 3(c). The master disk has a structure in which, as shown in the figure, soft magnetic films (from the cobalt family of soft magnetic films) 44 are embedded on a face of a silicon substrate 45 contacting the surface of the magnetic recording medium 40.
As shown in FIG. 4(b), the intervention of the substrate, embedded with patterns of the soft magnetic films, between the permanent magnet 43 and the magnetic recording medium 40, allows the leakage magnetic flux from the permanent magnet 43 that entered the silicon substrate 45 (the direction MFD of a magnetic field for writing printed signal is opposite to the direction of the magnetic field for demagnetization) to pass through the silicon substrate 45 again and magnetize the magnetic film 42 at a position without the soft magnetic film 44. However, at a part with presence of the soft magnetic pattern, the leakage magnetic flux passes through the soft magnetic film 44 so as to form a magnetic circuit with low magnetic resistance. As the magnet 43 is moved in the direction TD across the silicon substrate 45, this reduces the leakage magnetic flux from the silicon substrate 45 to a small amount at a position with the soft magnetic film 44, by which no writing by magnetization is carried out. With the mechanism as above, magnetic printing of the servo signal is carried out.
FIGS. 5(a) to 5(e) are views for explaining the process steps for manufacturing the master disk. FIG. 5(a) shows the step of applying a resist. FIG. 5(b) shows the step of patterning the magnetic pattern. FIG. 5(c) shows the step of etching the silicon substrate. FIG. 5(d) shows the step of sputtering soft magnetic material. FIG. 5(e) shows the step of lift-off. The respective steps are explained below.
First step: On the surface of a silicon substrate 51 (with a substrate thickness of about 500 μm), a resist 52 (with a thickness of 1.2 μm) is applied by using a spin coater (FIG. 5(a)). Thereafter, patterning is carried out on the resist 52 by employing an optical lithography method similar to that in a manufacturing method for a normal silicon semiconductor (FIG. 5(b)). The resist 52 is used as a mask for etching in a second step, and is formed of a material in the novolak family, which is not so strong against etching. Therefore, it is important for the resist 52 to have such a thickness that it does not disappear even when it is etched.
Second step: By employing a reactive plasma-etching method (reaction gas: methane trichloride), the silicon substrate 51 is subjected to dry etching to the depth of 500 nm (FIG. 5(c)).
Third step: By employing a sputtering method, deposition of a soft magnetic film 53 of Co (cobalt) is carried out to a thickness of 500 nm (FIG. 5(d)).
Fourth step: After the deposition of the Co soft magnetic film 53, the silicon substrate 51 is immersed in a solvent that dissolves the resist 52 (while employing ultra sound and the like as necessary), by which the resist 52 between the Co soft magnetic film 53 and the silicon substrate 51 is dissolved to remove it (FIG. 5(e)).
Japanese Official Gazettes disclosing prior art relating to the invention are described as follows.
The art described in a Japanese patent publication No. JP-A-2001-34938, relates to a master information carrier, by which high density information signals can be uniformly and stably recorded over the whole face of a magnetic recording medium, and a method of manufacturing the carrier. The carrier is provided with a substrate and a ferromagnetic thin film disposed on the substrate so as to form a pattern arranged to correspond to a magnetic pattern with the surface of the ferromagnetic film made approximately flat. Moreover, the art described in a PCT patent publication, WO 00/26904 relates to a master information carrier and a magnetic recording method that uses the carrier, in which a figure pattern corresponding to an arrangement of information signals for being recorded in a magnetic recording medium is provided by an arrangement of ferromagnetic thin films deposited on the surface of a non-magnetic substrate.
FIGS. 6(a) and 6(b) illustrate shapes of servo patterns. FIG. 6(a) shows a part of the servo pattern. In the figure, cross-hatched regions surrounded by closed curves (although the lines, being magnified in the figure, look like straight lines, they are actually curved lines) are areas where soft magnetic films are formed. FIG. 6(b) shows patterns of contents data. Similarly in this figure, cross-hatched regions surrounded by closed curves (although the lines, being magnified in the figure, look like straight lines, again they are actually curved lines) are areas in which soft magnetic films are formed.
Line widths of the patterns range from one to several lines. In order to carry out magnetic printing of the servo patterns, the same patterns as the servo patterns must be formed on a master disk as patterns of soft magnetic film. The thickness of the soft magnetic film is very important. An excessively thin film thickness causes magnetic saturation when a magnetic field to be printed is applied, which results in leakage of a magnetic flux to the magnetic recording media facing the soft magnetic film. This, in the worst case, causes inversion in magnetization to produce a problem that a region where the data should be “0” have data of “1”.
FIGS. 7(a) and 7(b), and 8(a) and 8(b) each show magnetic field strength distributions around the magnetic film of a magnetic recording medium at printing. FIG. 7(a) shows magnetic field strength distributions on the surface of a magnetic recording medium in the case of P=1.4, W=0.7 μm, where P is the pitch and W is the width, of the soft magnetic film. FIG. 7(b) shows magnetic field strength distributions on the surface of a magnetic recording medium in the case of P=0.4, W=0.2 μm. FIG. 8(a) shows magnetic field strength distributions on the surface of a magnetic recording medium in the case of P=0.7, W=0.35 μm. FIG. 8(b) shows magnetic field strength distributions on the surface of a magnetic recording medium in the case of P=0.2, W=0.1 μm.
Namely, FIGS. 7(a) and 7(b), and FIGS. 8(a) and 8(b) are views showing magnetic field strength distributions at the magnetic film of a magnetic recording medium with a printing magnetic field kept constant and the width (the pattern width) and the film thickness of the soft magnetic film of the master disk made to vary. When printing is carried out, the magnetic field strength must be 4 kOe (4000 Oersteds) or more in a region where magnetization is inverted and must be 2 kOe (2000 Oersteds) or less in a region where magnetization is not inverted. The thickness of the soft magnetic film for satisfying the conditions becomes as follows.                Pattern width: 0.7 μm→                    Thickness of the soft magnetic film: 0.20 μm or more                        Pattern width: 0.35 μm→                    Thickness of the soft magnetic film: 0.20 μm or more                        Pattern width: 0.2 μm→                    Thickness of the soft magnetic film: 0.075 μm or more                        Pattern width: 0.1 μm→                    Thickness of the soft magnetic film: 0.050 μm or more                        
When the pattern width of the servo pattern is from 0.1 to 0.7 μm, the thickness of the soft magnetic film must be 0.20 μm or more to avoid magnetic saturation in all of the regions. Practically, however, within film nonuniformity arising in a photo-process and deposition of the soft magnetic film must be taken into consideration. Therefore, the thickness of the soft magnetic film should be 0.30 μm or more. Different cross-sectional shapes, in which a Co soft magnetic film is embedded in a groove etched in a photo-process, are shown respectively in FIGS. 9(a) to 9(g), and a relationship between groove width and film thickness at the groove section is shown in FIG. 10.
FIG. 9(a) shows a 0.5 μm groove width section. FIG. 9(b) shows a 1.0 μm groove width section. FIG. 9(c) shows a 1.5 μm groove width section. FIG. 9(d) shows a 2.0 μm groove width section. FIG. 9(e) shows a 2.5 μm groove width section. FIG. 9(f) shows a 3.0 μm groove width section. FIG. 9(g) shows a 3.5 μm groove width section. In these figures, reference numeral 91 denotes a silicon substrate, reference numeral 92 denotes a resist, and reference numeral 93 denotes a soft magnetic film.
From FIGS. 9(a) to 9(f) and FIG. 10, it can be seen that as the groove width becomes smaller, thickness nonuniformity of the soft magnetic film deposited in the groove becomes noticeable. In FIG. 10, it is shown that the nonuniformity becomes non-negligible for groove widths of 1 μm or less. This presents a problem. That is, the narrow the pattern width becomes, the more difficult it becomes to embed the soft magnetic film in a deep groove. Therefore, a method of manufacturing a master disk has been long awaited, which facilitates magnetic printing even for groove widths of 1 μm or less.